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Re: How to capture signature using Canvas and save in png format?

You need to save the points used to draw the lines in some container like an arraylist so you can you use them when you want to save what was drawn on the jpanel.

Your method of drawing the image of lines is wrong. You need to override the component's paintComponent method and draw the lines there using what you have saved in the arraylist.
If you move the frame with the image past the side of the monitor, the drawn lines are erased. Or if you minimize and restore the window the image is gone.